Failure to keep the correct distance between two vehicles following one another is one of the principal factors leading to road accidents. Conforming the distance between adjacent vehicles to their speeds under variable conditions is subjective and depends on many factors. Thus, for example, lighting conditions, ambient weather, traffic density, road conditions, driver alertness and so on are all factors which can influence a driver's ability to keep a safe distance between two vehicles which, if not properly maintained, can result in a rear-end collision.
Existing anti-collision systems are either passive or active. Passive devices warn the driver of a rear, or following, vehicle when he is approaching too close to the vehicle in front (hereinafter the "leading" vehicle) for his current road speed. The warning having been provided, the initiative is now the driver's to take suitable precautionary action.
In contrast to this, active devices include a servo-control system typically coupled to the throttle valve so that, in the event that the driver does not maintain a safe distance from the leading vehicle for his current road speed, the following vehicle is automatically decelerated so as to reduce its road speed to a safe stopping speed based on the reduced distance between the two vehicles. U.S. Pat. No. 4,706,195 (Yoshino et al.) describes such an active speed control system.
The system disclosed by Yoshino et al. is based on a laser radar for measuring the distance of the following vehicle from the leading vehicle, the speed of the following vehicle being determined by a speed sensor coupled to the speedometer.
It is clear that passive systems also must comprise at least these two elements: namely some sort of rangefinder for determining the distance between two vehicles and also means for determining the speed of the following vehicle. However, in practice, it has been found that both passive and active systems have proven difficult to exploit commercially for several reasons. First, it is difficult to design a system which is easily amenable to coupling to all existing vehicles without requiring special customization for each vehicle. As soon as such customization is required, this raises the price of the system.
A further drawback relating to the development of such systems concerns the occurrence of false alarms. When a false alarm is obtained with active systems, the result is not merely irritating but can even be dangerous in that it is liable to lead to a rear-end collision between the following vehicle and the one behind it. This can occur because the following vehicle abruptly slows down (or stops) for no reason, leaving the vehicle behind him insufficient time to take precautionary action and thereby leading to a collision between the two vehicles.
In passive systems, false alarms will produce similar dangerous results if the driver panics as a result of the alarm, applying his brakes too abruptly and resulting possibly in skidding of his own vehicle or, for the same reasons as explained above, a rear-end collision between his vehicle and the one behind.
On a straight, level road with no camber and a uniformly distributed load in the following vehicle, it is relatively easy to design laser rangefinders which produce reliable and accurate results. However, in practice, such ideal conditions are rarely met. A non-uniform load distribution in the following vehicle can sufficiently alter the angle of inclination of the laser beam to produce erroneous results even on a level road surface. Thus, for example, people sitting on the rear seat or the addition of heavy luggage in the trunk of the vehicle can produce incorrect results in prior art systems. Likewise, as the following vehicle adjusts the steering angle in order to turn a curve, the direction of the laser beam, which remains coaxial with the longitudinal axis of the following vehicle, can miss a leading vehicle in the same lane or can strike a vehicle in an adjacent lane.
Yet a further consideration is how the speed of the following vehicle is determined. In vehicles having digital speedometers, it is straightforward to extract the digital readout thereof for use in a digital anti-collision system. However, many existing motor vehicles still employ analog speedometers, in which case it is necessary to extract the analog output thereof and convert it to an equivalent digital signal for use with a digital anti-collision system. As indicated above, this requires customization since not all analog speedometers are identical. Such customization increases the price of the system and may well render it commercially unacceptable.
Yet a further drawback associated with existing anti-collision devices is that the rangefinder is usually mounted outside of the vehicle typically near the fender, in accordance with the rationale that this is the initial point of contact with the leading vehicle in the event of a collision. However, mounting the rangefinder outside of the vehicle renders it susceptible to ambient weather conditions, to atmospheric pollution including the exhaust gases of the leading vehicle, and indeed to dirt which settles on the rangefinder's optics, thereby causing distortions and inaccuracies.
Yet a further consideration relates to the manner in which the device presents data to the driver. The alarm itself is, of course, typically audible. However, no less important than the alarm itself, is the calculated collision time between the two vehicles and whose rate of change is an important guide in preserving a safe stopping distance even before any alarm signal is sounded. If such data is presented to the driver via an instrument on the dashboard, this requires that the driver look down at the dashboard, thereby momentarily taking his eyes off the road. This is particularly hazardous in those situations where there exists a high probability of a collision. It is precisely in such situations that a driver must be completely aware of prevailing road conditions without, even momentarily, losing sight of the leading vehicle.